1. Field of the Invention
The invention relates to signal modulation, and in particular to harmonic-rejection modulation devices which reduce the harmonics of local oscillating signals to improve the modulation quality.
2. Description of the Related Art
In wireless communication systems, up-conversion architecture of a transmitter includes a modulator. Based on local oscillating signals, the modulator converts a baseband signal to an intermediate frequency signal. Sideband suppression, carrier suppression, and undesired harmonic suppression are criteria for judging the modulation quality. Sideband suppression relates to sideband leakage caused by phase or amplitude errors of modulating signals. Carrier suppression relates to carrier leakage caused by dc offset of modulating signals. Undesired harmonic suppression relates to undesired harmonics generated in the modulation process.
A LC filter circuit coupled to the modulator can filter undesired harmonics, but the filter circuit occupies a large circuit area and increases production costs. Another solution is reduction of specific harmonics by mathematical operation in modulation process. FIG. 1 shows a conventional modulation device 100 including a first poly-phase filter 102, a second poly-phase filter 104, two amplifiers 106 and 108, a modulator 110, and a filter circuit 130. An input signal 112 of the modulation device 100 is a sine wave. Based on the input signal 112, the first poly-phase filter 102 generates four signals (114) having phase angles of 45°, 135°, 225° and 315°, respectively. Based on signals 114, the second poly-phase filter 104 generates four signals 116 and four signals 118. Signals 116 have phase angles of 30°, 90°, 210°, and 270°, respectively. Signals 118 have phase angles of 0°, 120°, 180°, and 300°. Based on signals 116, the amplifier 106 generates signals 120. Based on signals 118, the amplifier 108 generates signals 122. Signals 120 and 122 are local oscillating signals for the modulator 110. Based on the local oscillating signals 120 and 122, the modulator 110 modulates the in-phase signal (I) and the quadrature phase signal (Q) of a baseband signal. The third harmonics of the local oscillating signals 120 and 122 are eliminated by mathematical operations of the modulator 110. The filter circuit 130 filters high-order harmonics of the output signals (124 and 126) of the modulator 110.
Drawbacks occur, however, when utilizing the modulation device 100. The input signal 112 of the modulation device 100 is limited to a sine wave. The modulation device 100 is incapable of dividing the frequency of the input signal 112 and an additional circuit for dividing frequency of signals is required. High current levels are required to control the amplitude of the local oscillating signals. The filer circuit 130, which filters the high-order harmonics of the signals 124 and 126, occupies a large area, increasing production costs and energy consumption.